Transform Java Future into a CompletableFuture

Question

Java 8 introduces CompletableFuture, a new implementation of Future that is composable (includes a bunch of thenXxx methods). I\'d like to use this exclusively,

Answer 1

If the library you want to use also offers a callback style method in addition to the Future style, you can provide it a handler that completes the CompletableFuture without any extra thread blocking. Like so:

    AsynchronousFileChannel open = AsynchronousFileChannel.open(Paths.get("/some/file"));
    // ... 
    CompletableFuture completableFuture = new CompletableFuture();
    open.read(buffer, position, null, new CompletionHandler() {
        @Override
        public void completed(Integer result, Void attachment) {
            completableFuture.complete(buffer);
        }

        @Override
        public void failed(Throwable exc, Void attachment) {
            completableFuture.completeExceptionally(exc);
        }
    });
    completableFuture.thenApply(...)

Without the callback the only other way I see solving this is to use a polling loop that puts all your Future.isDone() checks on a single thread and then invoking complete whenever a Future is gettable.